In an analog FM (frequency modulation) stereo radio system, the left channel (L) and right channel (R) of the audio signal are conveyed in a midside (M/S) representation, i.e. as mid channel (M) and side channel (S). The mid channel M corresponds to a sum signal of L and R, e.g. M=(L+R)/2, and the side channel S corresponds to a difference signal of L and R, e.g. S=(L−R)/2. For transmission, the side channel S is modulated onto a 38 kHz suppressed carrier and added to the baseband mid signal M to form a backwards-compatible stereo multiplex signal. This multiplex signal is then used to modulate the HF (high frequency) carrier of the FM transmitter, typically operating in the range between 87.5 to 108 MHz.
When reception quality decreases (i.e. the signal-to-noise ratio over the radio channel decreases), the S channel typically suffers more than the M channel. In many FM receiver implementations, the S channel is muted when the reception conditions gets too noisy. This means that the receiver falls back from stereo to mono in case of a poor HF radio signal.
Parametric Stereo (PS) coding is a technique from the field of very low bitrate audio coding. PS allows encoding a 2-channel stereo audio signal as a mono downmix signal in combination with additional PS side information, i.e. the PS parameters. The mono downmix signal is obtained as a combination of both channels of the stereo signal. The PS parameters enable the PS decoder to reconstruct a stereo signal from the mono downmix signal and the PS side information. Typically, the PS parameters are time- and frequency-variant, and the PS processing in the PS decoder is typically carried out in a hybrid filterbank domain incorporating a QMF bank. The document “Low Complexity Parametric Stereo Coding in MPEG-4”, Heiko Purnhagen, Proc. Digital Audio Effects Workshop (DAFx), pp. 163-168, Naples, IT, October 2004 describes an exemplary PS coding system for MPEG-4. Its discussion of parametric stereo is hereby incorporated by reference. Parametric stereo is supported e.g. by MPEG-4 Audio. Parametric stereo is discussed in section 8.6.4 and Annexes 8.A and 8.C of the MPEG-4 standardization document ISO/IEC 14496-3:2005 (MPEG-4 Audio, 3rd edition). These parts of the standardization document are hereby incorporated by reference for all purposes. Parametric stereo is also used in the MPEG Surround standard (see document ISO/IEC 23003-1:2007, MPEG Surround). Also, this document is hereby incorporated by reference for all purposes. Further examples of parametric stereo coding systems are discussed in the document “Binaural Cue Coding—Part I: Psychoacoustic Fundamentals and Design Principles,” Frank Baumgarte and Christof Faller, IEEE Transactions on Speech and Audio Processing, vol 11, no 6, pages 509-519, November 2003, and in the document “Binaural Cue Coding—Part II: Schemes and Applications,” Christof Faller and Frank Baumgarte, IEEE Transactions on Speech and Audio Processing, vol 11, no 6, pages 520-531, November 2003. In the latter two documents the term “binaural cue coding” is used, which is an example of parametric stereo coding.
Even in case the mid signal M is of acceptable quality, the side signal S may be noisy and thus can severely degrade the overall audio quality when being mixed in the left and right channels of the output signal (which are derived e.g. according to L=M+S and R=M-S). When a side signal S has only poor to intermediate quality, there are two options: either the receiver chooses accepting the noise associated with the side signal S and outputs real stereo, or the receiver drops the side signal S and falls back to mono.